The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon.
The invention relates generally to ship roll stimulation, and more particularly to a method of quantifying design parameters for a ship roll stimulation system.
Understanding roll motion for a particular type of ship is important for planning the loading/unloading of the ship as well as how the ship will react in heavy weather. Since inherent characteristics vary from ship to ship, testing of ship roll must be carried out on a ship-by-ship basis. Currently, the most widely-used methods of generating roll motion include sallying, snap load release, waiting for weather conditions to induce roll, and the use of motion tables. Sallying involves running a number of sailors across the deck of a ship. While useful for quickly checking the natural frequency period of the ship, sallying is not a consistent sustainable method for producing roll motion and is not a cost effective use of personnel. In snap load release methods, a ship with a crane hoists a heavy load over side and xe2x80x9csnapxe2x80x9d releases the load. This method is effective for measuring righting moment, natural frequency and roll damping, but cannot be used for consistent roll generation. Furthermore, snap releasing of the load is stressful on the equipment and can be risky for personnel owing to the large weights used. The study of roll motion that relies on weather conditions to induce roll is the more traditional method, but is inconsistent, highly unreliable and not a cost effective use of personnel or equipment. Motion tables are typically hydraulically driven xe2x80x9cdecksxe2x80x9d that can simulate all of the various ship motions. However, they are limited in size and power, very expensive to build and operate, and do not provide a realistic operating environment for use in training.
Other more sophisticated ship roll stimulation systems include: moving mass systems using counter rotating weights, a weighted cart or active flume tanks; force systems using ballast tanks or weighted anchors; and torque systems using pivoting gyroscopes.
In moving mass systems, the transfer of weight across the breadth of the ship creates the moment, or torsion, used to generate ship roll. The counter rotating weight system utilizes two equally weighted carts synchronously rotating about two shafts in opposing directions. The weighted cart system involves linearly moving a massive cart back and forth across the ship centerline. The active flume tank system uses a U-shaped tank, typically filled with water, and pushes the fluid back and forth.
In force systems, a force is generated at some distance from the ship centerline to generate a moment. For example, ballast tanks can be placed on the outboard flanks of a ship and have pressurized air cycled in and out of the tanks to push water and generate buoyant forces weighted anchors can be placed on each side of the ship and have winches selectively pull on one anchor and then the other to create the cycled forces needed.
In a torque system using a pivoting gyroscope, torque can be created directly by rotating and counter-rotating the gyroscope against the inertial mass of the spinning flywheel. Each of these systems and numerous others can be used to create the moment required to generate ship roll, however, there has been no reliable scientific method previously developed for sizing or quantifying the various design parameters of each of these systems for the purpose of stimulating consistent, repeatable roll. Rather, trial and error methodologies have governed ship roll stimulation system design leading to inconsistent and often non-repeatable or very limited roll motion testing.
Accordingly, it is an object of the present invention to provide a method that aids in the design of a ship roll stimulation system for a particular ship.
Another object of the present invention is to provide a method for quantifying design parameters for a ship roll stimulation system.
Still another object of the present invention is to provide a simple method for quantifying design parameters for any moving mass ship roll stimulation system.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is provided for quantifying design parameters for a ship roll stimulation system. A ship to be tested has a maximum angle of roll xcex8MAX that needs to be induced. A plurality of inherent ship parameters that are provided (or determined) include a righting moment RM per degree of list, the ship""s natural roll period Tn, a damping ratio xcex6 and a ship""s center of gravity. For a particular moving mass ship roll stimulation system which has been selected, a total induced moment expression at the ship""s natural roll period Tn is determined. The total induced moment expression includes a primary moment component and a secondary moment component that are functions of a plurality of design parameters for the moving mass ship roll stimulation system. The total induced moment expression is equated to 2xcex6xcex8MAXRM and values for the design parameters are selected to satisfy the equality.